FN-5796: make plugin same-state transitions idempotent

Prevent duplicate-state plugin updates from throwing invalid transition errors.

- treat updatePluginState calls where next state equals current state as idempotent no-ops
- allow same-state updates with an explicit error payload to persist error changes and emit plugin:updated
- add regression tests for same-state no-op behavior, no extra stateChanged event emission, and error payload updates
- add a patch changeset documenting the Dependency Graph plugin fix

Files changed:
 .changeset/fn-5796-plugin-state-noop.md          |  7 +++++
 packages/core/src/__tests__/plugin-store.test.ts | 34 ++++++++++++++++++++++++
 packages/core/src/plugin-store.ts                | 15 +++++++++++
 3 files changed, 56 insertions(+)

Fusion-Task-Id: FN-5796

Fusion-Task-Lineage: 0da1d035-383f-456c-a212-0903dc029874

packages/core/src/__tests__/plugin-store.test.ts

@@ -663,6 +663,40 @@ describe("PluginStore", () => {
       ).rejects.toThrow("Invalid state transition");
     });
 
+    it("treats same-state transitions as no-op", async () => {
+      const manifest = makeManifest();
+      await store.registerPlugin({ manifest, path: "/path/to/plugin" });
+      await store.updatePluginState("test-plugin", "started");
+
+      await expect(store.updatePluginState("test-plugin", "started")).resolves.toMatchObject({
+        id: "test-plugin",
+        state: "started",
+      });
+    });
+
+    it("does not emit plugin:stateChanged for same-state transitions", async () => {
+      const stateChanged = vi.fn();
+      store.on("plugin:stateChanged", stateChanged);
+
+      const manifest = makeManifest();
+      await store.registerPlugin({ manifest, path: "/path/to/plugin" });
+      await store.updatePluginState("test-plugin", "started");
+      expect(stateChanged).toHaveBeenCalledTimes(1);
+
+      await store.updatePluginState("test-plugin", "started");
+      expect(stateChanged).toHaveBeenCalledTimes(1);
+    });
+
+    it("updates error on same-state transition when error payload is provided", async () => {
+      const manifest = makeManifest();
+      await store.registerPlugin({ manifest, path: "/path/to/plugin" });
+      await store.updatePluginState("test-plugin", "started");
+
+      const plugin = await store.updatePluginState("test-plugin", "started", "Recovered warning");
+      expect(plugin.state).toBe("started");
+      expect(plugin.error).toBe("Recovered warning");
+    });
+
     it("allows restarting from stopped", async () => {
       const manifest = makeManifest();
       await store.registerPlugin({ manifest, path: "/path/to/plugin" });

packages/core/src/plugin-store.ts

@@ -487,6 +487,21 @@ export class PluginStore extends EventEmitter<PluginStoreEvents> {
       throw new Error(`Invalid state: ${state}`);
     }
 
+    if (state === oldState) {
+      // Same-state transitions are idempotent by design. Only emit plugin:updated
+      // when a provided error payload actually changes persisted plugin fields.
+      if (error === undefined || plugin.error === error) {
+        return plugin;
+      }
+
+      this.upsertProjectState(id, { state, error });
+      this.centralDb.bumpLastModified();
+
+      const updated = await this.getPlugin(id);
+      this.emit("plugin:updated", updated);
+      return updated;
+    }
+
     if (state !== "error") {
       const validTransitions: Record<PluginState, PluginState[]> = {
         installed: ["started", "stopped", "error"],